Abstract: The black hole (BH) candidate SWIFT J1753.5-0127 has remained active since
the onset of its 2005 outburst. Emission lines in the optical spectrum were
observed at the very beginning of the outburst, but since then the spectrum has
been featureless making a precise BH mass estimation impossible. Here we
present results from our optical and UV observations of SWIFT J1753.5-0127
taken in 2012-2013. Our new observations show extremely broad, double-peaked
emission lines in the optical and UV spectra. The optical data also show narrow
absorption and emission features with nearly synchronous and significant
Doppler motions. A radial velocity study of these lines which we associate with
the secondary star, yields a semi-amplitude of K_2=382 km/s. A time-series
analysis of the spectral and photometric data revealed a possible orbital
periodicity of 2.85 h, significantly shorter than the reported 3.2 h periodic
signal by Zurita et al. The observed variability properties argue against a low
orbital inclination angle and we present several observational arguments in
favour of the BH interpretation. However, the measured radial velocity
semi-amplitude of the donor star and the short orbital period imply that SWIFT
J1753.5-0127 has one of the lowest measured mass function for a BH in a
low-mass X-ray binary. We show that the compact object mass in excess of 5 Msun
is highly improbable. Thus, SWIFT J1753.5-0127 is a BH binary that has one of
the shortest orbital period and hosts probably one of the smallest stellar-mass
BH found to date.